This invention relates to a process for recovery of waste H.sub.2 SO.sub.4 and HCl which are used for pickling metallic materials and articles and contain large quantities of Fe ions. Furthermore, it offers a process for recovery H.sub.2 SO.sub.4 and HCl from waste H.sub.2 SO.sub.4 which contains large amounts of Fe ions after removal of Ti ions by a hydrolysis process in the production of TiO.sub.2 and also the recovery of high-purity electrolytic iron or iron hydroxide.
There are two conventional methods of acid recovery. The first method is a crystallization method which crystallizes Fe ions contained as their sulfate or chloride salt by cooling or concentrating waste acids with heat. However, this method has the following disadvantages: (1) large amounts of energy are required for the crystallization; (2) the removal of heavy metallic ions, such as, Fe ions from the waste acids is difficult; (3) an economical method for treating the crystals removed is not available.
The second method is a thermal decomposition method which sprays the waste acids, including Fe ions, into a furnace heated from 600.degree. to 1000.degree. C. The HCl or H.sub.2 SO.sub.4 is recovered by absorbing the gases decomposed into the forms of HCl gas, Cl.sub.2 gas and Fe.sub.2 O.sub.3 or the forms of H.sub.2 SO.sub.4 gas, SO.sub.2 gas, SO.sub.3 gas and Fe.sub.2 O.sub.3, and these are reused for pickling and dissolution of raw materials. The following disadvantages are found in this method. (1) a large energy loss for thermal decomposition in the case of lower Fe ions concentration in the waste acids; (2) an abundant hydrochloric acid loss as HCl fume unless the total Cl.sup.- ions concentration in the pickling process is increased; (3) beating of the pickling solution required to raise the Fe ions concentration in it; (4) difficulty in controlling the Fe concentration in practice, and (5) a low operating rate and high maintenance cost based on severe damage of the apparatus by wet Cl.sub.2 and HCl gases.
Moreover, many methods which recover HCl or H.sub.2 SO.sub.4 by removal of Fe ions in the waste acids as electrolytic iron by electrolysis have been published. These methods introduce the waste HCl or H.sub.2 SO.sub.4 including abundant Fe ions into the cathode compartment in electrolysis process, electrodeposits of Fe metal on the cathode, and at the same time transfer Cl.sup.- or SO.sub.4.sup.2- ions through the anion exchange membrane which divides the cathode and central compartments, and recycle the waste acids which having passed the cathode compartment into the used parts through the central compartment. Abundant free acids in the waste acid in the electrolysis process yield a low Fe electrolysis efficiency, because of the decomposition to H.sup.+ and Cl.sup.- ions or H.sup.+ and SO.sub.4.sup.2- ions (see FIG. 14).
To overcome this disadvantage, the conventional method has proposed to set the diffusion-dialysis membrane prior to the electrolysis process and decrease the amounts of free acid as possible. However, the above method cannot improve the relationship of the increasing amounts of free acid attended by electrodeposition of Fe metals due to the direct electrolysis. Furthermore, since the waste acids often contain the heavy metallic ions except Fe ions, only a poor-purity electrolytic iron is obtained in the electrolysis process and consequently high additional valuable materials cannot be obtained.